Load control devices, such as dimmer switches and dimming modules, for example, may be configured to control an amount of power provided from an alternating current (AC) power source to a load, such as a lighting load, for example. Such load control devices may employ a bidirectional semiconductor switch that is coupled in series electrical connection between the AC power source and the load. The bidirectional semiconductor switch may be controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source, for example, to control the amount of power delivered to the load (e.g., using a phase-control dimming technique). For example, the bidirectional semiconductor switch may comprise one semiconductor switch, such as, but not limited to a triac or a field effect transistor (FET) within a full-wave rectifying bridge; two semiconductor switches, such as, but not limited to two FETs or two insulated gate bipolar transistors (IGBTs), coupled in anti-series electrical connection, or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel electrical connection.
Load control devices may use a forward phase-control dimming technique or a reverse phase-control dimming technique, for example, to control when the bidirectional semiconductor switch is rendered conductive and non-conductive to control the power delivered to the load. During forward phase-control dimming, the bidirectional semiconductor switch may be turned on at some point within each AC line voltage half-cycle and remains on until the next voltage zero crossing. Forward phase control dimming may be used to control the power delivered to a resistive or inductive load, which may include, for example, an incandescent lamp or a magnetic low-voltage transformer, respectively. During reverse phase-control dimming, the bidirectional semiconductor switch may be turned on at the zero crossing of the AC line voltage and turned off at some point within each half-cycle of the AC line voltage. Reverse phase-control dimming may be used to control the power delivered to a capacitive load, which may include, for example, an electronic low voltage transformer. Given that the bidirectional semiconductor switch may be rendered conductive at the beginning of the half-cycle and may be able to be turned off within the half-cycle, reverse phase control dimming may be utilized by a dimmer switch that includes two FETs in anti-serial connection, or the like.
Load control devices may be programmed by a user during installation to use the reverse phase-control dimming technique or the forward phase-control dimming technique during operation. Alternatively, load control devices may employ a load detection process wherein the load control device may determine the type of load that it is controlling and use the phase-control dimming technique that is best suited for that load type. For example, a load control device may detect that the load is inductive, and may determine to use the forward phase-control dimming technique. For example, upon initial power up, such a load control device may begin using a reverse phase-control dimming technique (e.g., operating in reverse phase-control mode) and may monitor the voltage across the load during the load detection process. In the event that the load control device detects an overvoltage condition (e.g., a voltage spike), the load control device may then determine that the load has inductive characteristics, and may accordingly begin using a forward phase-control dimming technique (e.g., operating in forward phase-control mode). After a load control device determines to employ a phase-control technique (e.g., forward phase-control dimming technique), for example, in response to a load detection process, user programming, or as a result of pre-configuration at the factory), the load control device may continue to use the determined phase-control technique during operation and may not deviate from using the determined phase-control technique.
However, a load control device may achieve improved performance as a result of deviating from a phase-control technique (e.g., a forward phase-control dimming technique) to employ another phase-control technique (e.g., a reverse phase-control dimming technique) during certain conditions. Therefore, there is a need for an improved load control device that is operable to employ one phase-control technique (e.g., a forward phase-control dimming technique) during operation and employ another phase-control technique (e.g., a reverse phase-control dimming technique) during certain conditions.
Further, some electrical loads, such as a compact fluorescent lamp (CFL) or a light emitting diode (LED) lamp, for example, may comprise a capacitor (e.g., a bus capacitor). If the capacitor is not charged, then the use of a forward phase-control diming technique may cause a current spike, which, for example, may occur at start-up when the capacitor is fully dissipated. This current spike may be due to the relatively large change in voltage across the capacitor at a given time (e.g., an instantaneous voltage across the capacitor). However, the load coupled to the load control device may be best suited for a forward phase-control dimming technique. Therefore, there is a need for an improved load control device that may be operable to employ a reverse phase-control dimming technique and/or a center phase control technique to charge the capacitor, and to employ a forward phase-control dimming technique to operate the load.
Additionally, an electrical load, such as a light emitting diode (LED) lamp, for example, may require a certain minimum voltage across it in order to turn on (e.g., emit light). However, once turned on, the load may be able to operate with an even lower voltage (and thus provide a lower light intensity) than is required to turn the lamp on. Therefore, a load control device may be forced to select between operating a load with the lowest possible light output and guaranteeing that the load will turn on when the certain minimum voltage is being applied. Thus, there is a need for an improved load control device that is operable to employ a reverse phase-control technique, a center phase-control technique, and a forward phase-control dimming technique in a single operation, for example, in order to charge a capacitor, provide sufficient voltage across a load to turn it on, and operate the load while allowing it to reach its lowest possible light output (low-end).